Geomagnetic storms in the Antarctic F-region. II. Physical interpretation

The physical explanation of the changes of the maximum electron concentration of the F2 layer, NmF2. as a result of geomagnetic disturbances has proved elusive even after several decades of study. In this paper, we concentrate upon the main phase of the F-region storm which usually begins a few hours after the onset of a significant increase of geomagnetic activity. We present some new analysis of ionosonde data from the South Atlantic and Antarctica. There are major differences in storm phenology in the South compared with the North. In winter in the South, NmF2 is usually increased as the result of a geomagnetic storm whereas in the North it is depressed. In summer, both hemispheres normally show reduced values of NmF2 as a result of storm activity. It is these features which provide new insight into the problem of F-region storms. We interpret the observed seasonal and diurnal variation of NmF2 in terms of a mean (DC) effect and a diurnal variation (AC) effect, and discuss possible explanations for both. We conclude that there must be direct energy input at mid-latitudes during geomagnetic storms to cause the observed DC effect. We suggest that the energy sources are the precipitation of ring current ions, particularly O+, and the penetration of the cross-tail magnetospheric electric field to mid-latitudes in the first few hours of the storm. We attribute the AC effect to changes in the magnitude and direction of the thermospheric wind caused by alterations in momentum exchange between the neutral and ionized gas (ion drag). We suggest that longitudinal asymmetries in the DC sources may also contribute to the AC effect. Our interpretation and conclusions are consistent with previous in situ observations at mid-latitudes in which changes of the ON2 ratio are observed. Further three-dimensional modelling of the coupled ionosphere and thermosphére, combined with additional F-region neutral wind observations at mid-latitudes, are necessary to test our suggestions.

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